1. Field of the Invention
The present invention relates to a hot electron device (HET), and particularly to a hot electron device in which an indium arsenide (InAs) epitaxlal layer which has high electron mobility due to small effective electron mass is specially formed as a base layer material in a hetero structure hot electron device.
The present invention also relates to a resonant tunneling hot electron device which induces a resonant tunneling by adding an emitter electron projection layer to the hot electron device.
For several years, developments of semiconductor devices using the heterostructure have been vigorous' as the semiconductor growth technology such as the molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) has been developed. Among the devices, the hot electron device using very short and quick transition time of hot electron in the base region has been given a great interest and the research thereof have been under progress. In case of using an indium arsenide having high electron mobility as the base material, the electron transition time is further improved so that the performance of the device is improved.
The conventional techniques that improve the electrical characteristics of the base layer are as follows:
1) n-GaAs (emitter) /i-AlGaAs (emitter barrier layer) /n-GaAs (base) /i-AlGaAs (collector barrier layer) /n-GaAs (collector) /GaAs (substrate);
2) n-GaAs (emitter) /i-AlGaAs (emitter barrier layer) /n-InGaAs (base) /i-AlGaAs(collector barrier layer) /n-GaAs (collector) /GaAs (substrate);
3) n-InGaAs (emitter) /i-InAlAs (emitter barrier layer) /n-InGaAs (base) i-InAlAs (collector barrier layer) /n-InGaAs (collector) /InP (substrate);
4) n-AlSbAs (emitter) /n-InAs (base) /i-GaSb (collector barrier layer) /n-GaSb (collector) /GaSb (substrate).
Classifying the hot electron device according to the technique which can improve the performance of the device due to the improvement of the base layer, the device can be classified into a gallium arsenide based (GaAs-based) ,indium phosphide based (InP-based) and gallium antimonide based (GaSb) series. In case of GaAs-based series hot electron device, a doped gallium arsenide layer is used as an emitter, base and collector layers. An aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As), x=0.3-1.0, is used as an emitter barrier layer between the emitter layer and base layer and as a collector barrier layer between the base layer and collector layer. In case of using a doped gallium arsenide layer as the base layer, the ratio of current that can move from the emitter to collector is small so as to lower the height of the collector barrier layer so that the collector current gain is raised. However, as the height of the collector barrier layer is lowered, the collector-base voltage V.sub.CB with which the operation of device is possible without leak current of collector layer is lowered. Therefore, high electron mobility indium gallium arsenide (In.sub.x Ga.sub.1-x As), x=0.53-0.8, is introduced as the base layer to improve the electrical characteristics. In this case, .GAMMA.-L separation is large so that dispersion of electrons due to L valley is reduced and the conduction band discontinuity with collector barrier layer is increased thereby reducing the content of aluminum arsenide AlAs of collector barrier layer and raising the collector-base voltage V.sub.CB. An indium gallium arsenide (In.sub.x Ga.sub.1-x As) base layer of indium composition of 0.53 to 0.8 is lattice mismatched with gallium arsenide layer. The lattice mismatch causes strain and crystal defects in the lattice thereby raising device operation voltage. To solve this problem, it is necessary to reduce the thickness and doping concentration of base layer, or to lower the height of collector barrier layer. However, as the doping concentration of base is reduced, scattering within the base layer reduces and current gain increases, resulting in increase in device ohmic resistance. It is known that, in general, an indium arsenide layer can lower base resistance and raise current gain at the same doping concentration compared with indium gallium arsenide (In.sub.x Ga.sub.1-x As), x=0.53-0.8. For example, ohmic resistance can be improved by lowering the height of Schottky barrier layer by introducing an indium arsenide layer at the surface. However, about 7% lattice mismatch exists between a gallium arsenide layer with a 5.6532 .ANG. lattic constant and an indium arsenide layer with a 6.0583 .ANG. lattice constant. The critical thickness which can grow due to the lattice mismatch exists, and the degradation of electrical characteristics occurs became of misfit dislocations, stacking faults, etc., at the grown epitaxial layer. Because of such problems, the indium phosphide material family is used wherein an InP substrates can accomplish lattice match.
In case of indium phosphide series hot electron device, doped indium gallium arsenide (InGaAs) is used as emitter, base and collector layers. An indium alluminium gallium arsenide (InAlGaAs) layer of a variety of aluminum composition is used as the emitter barrier layer between the emitter layer and base layer and as the collector barrier layer between the base layer and collector layer. Also in this case, since indium arsenide material having high electron mobility is applied, about 4% lattice mismatch exists between the indium phosphide layer with 5.8687 .ANG. lattice constant and indium arsenide layer with 6.0583 .ANG. lattice constant. There are various problems regarding critical thickness which can grow without crystal defects from lattice mismatch and the strain in an epitaxial layer. When growing the epitaxial layer, there is no problem if the layer is grown within its critical thickness, and electron transition time can be improved by reducing base layer resistance. This is accomplished base layer by reducing thickness, however, a significant problem arises since the device may be destroyed by metal diffusion at the time of forming ohmic contact to the base layer after growth of the epitaxial layer. Therefore, the base layer of thickness of about 300 .ANG. is necessary.
In case of gallium antimonide series, when an indium arsenide layer is applide, there is no substantial lattice mismatch between the gallium antimonide layer of unit length of crystal structure of 5.8687 .ANG. and indium arsenide layer of that of 6.0583 .ANG., therefore theoretically the lattice match is obtained and the epitaxial growth is possible. However, there were various problems in the manufacture of actual device due to difficulty in manufacture of high cost gallium antimonide substrate, unestablishment of thin film growth technology, and instability of device manufacturing process and soon.